In recent years, with rapid expansion of radio communication, insufficiency of radio communication bands is becoming an issue. A system for sharing frequencies in terms of time and location such as cognitive radio has been proposed to resolve the shortage of frequencies. Given the increasing traffic of radio communication, however, the system is not used sufficiently widely.
Consequently, there is an increasing demand for beam-forming as a technique for improving the efficiency of the use of radio waves (frequencies) in terms of space. Beam-forming is a technique of giving directivity to the radio waves to be radiated, thereby radiating the radio waves only toward a particular direction where the receiver is. This improves signal quality and reduces unnecessary radiation to other radio devices and systems. In other words, beam-forming allows the use of radio waves in a more spatially divided manner.
One of the typical beam-forming techniques is a phased array antenna. A phased array antenna changes the phases of radio signals fed to a plurality of regularly arranged antenna elements. A phased array antenna thereby spatially combines radio waves radiated from the antenna elements and radiates the combined radio waves to a desired direction. A phased array antenna radiates radio waves to a desired direction by adjusting the electric phases and amplitudes. Therefore, phased array antennas are more durable than mechanically operated high directional antennas. Beam-forming requires, however, that the direction of the receiver should be known as a prior condition. One of the simplest techniques for specifying the direction of the receiver is a beam-former method, which employs a device that scans the radio waves that it radiates. Techniques with higher precision for direction-of-arrival estimation include Minimum Mean Square Error (MMSE) and Multiple Signal Classification (MUSIC) methods. The MMSE method uses a known signal such as a preamble included in the signals to change the phases and the weightings of amplitudes in an array antenna, thereby giving a desired directivity to the radio waves to be radiated. The MUSIC method computes separation and direction of arrival based on eigenvalues and eigenvectors of correlation values of a received signal. This enables direction-of-arrival estimation of a received signal even when it is an unknown signal. Precisely speaking, the MMSE method is not for direction-of-arrival estimation but is a technique called adaptive array for optimizing the phases and weightings of amplitude for each antenna element. Since both the MMSE and MUSIC methods require high precision operations by digital signal processing, the signals received by the antenna elements need to be converted to digital signals by analog-digital (A/D) converters. As the number of antenna elements increases, the number of A/D converters proportionally increases, resulting in power consumption and cost increases.
To address the increase in the number of A/D converters, PTL1 discloses a technique for performing a kind of parallel-serial conversion in which signals can be serially inputted to a single circuit by providing each antenna element with a delay line with a different delay amount and a switch. PTL2 discloses a time-division phased array technique in which time-division multiplexing is performed by using switches but not using delay lines, thereby reducing the number of analog circuits to be connected.